Recently, digital electronic instruments are rapidly progressed in various field together with the development of the semiconductor industrials, especially digital electronic instruments for OA (office automation), FA (factory automation), cars, CD (compact disk), DAT (digital audio tape) and video tape are more rapidly progressed.
In general, these digital electronic instruments are commonly generated an electronic noise, and the noise may cause an electro magnetic interference (EMI, it is simply called as "noise" by any person) and the EMI may make mulfunctions or effects of reducing the S/N ratio of other hearby electronic equipment or appliance through the supply line or electrical radiation.
Therefore, the like noise is recently become a social problem and it is proceeding to restrict the diffusion of the noise under the legal in the world. For examples, to restrict the noise, under the FCC (federal communication commission) of the United States of America Docket No. 20780 was issued on October, 1979 and also administerred under the CSA in Canada and the VDE in Germany.
A filter is utilized as an electronic part to protect or reduce such noise and the generally characteristics of the filter are illustrated in FIGS. 1(A), 1(B) and 1(C), wherein FIG. 1(A) shows to the one of a simple or coil filter, FIG. 1(B) shows to the one of a L-type filter and FIG. 1(C) shows to the one of .pi.-type or T-type filter.
Hitherto, the three-terminal capacitor is usually used to protect or reduce a noise in low-frequency and the two-terminal capacitor or coil is often used to protect or reduce a noise in high-frequency. However they have shown many problems such as, for examples, occuring the residual inductance based on the lead wire and electrode structure, enlarging the suspended capacity inter wire wound, appearing the resonance point from several MHz to several tens MHz, and rapidly decreasing the effect of reducing the noise in high-frequency. Additionally, a lead through capacitor which does not occur the residual inductance in high-frequency may be applied, however it has the complexity in preparing them and increased the unit cost of production thereof. Accordingly, a lead through capacitor can be only applied to use restrictively [Refs.: E.P. Pat. Publn. Nos. 275093 and 276684, U.S. Pat. No. 4,370,698 and K.R. Pat. Publn. No. 90-2514 etc.]
Recently, it does not described in the literatures but a filter is practically made of combination of ferrite core and lead wire. Bead cores are often used as a component for a counter-noise for preventing EMI (electro magnetic interference). In the cases, ferrite cores having a small eddy current loss and a high magmetic permeability are also used, because ferrite cores are very cheap and it can be made a noise filter having a relatively structure and few side effects such as, distorted signals and extraordinary oscillation. These known filters consisted of ferrite core and lead wire are illustrated in FIGS. 2(A) to 2(D). A filter illustrated in FIG. 2(A) is an axial type one and filters illustrated in FIGS. 2(B), 2(C) and 2(D) are radial type one. As shown in FIGS. 3(A) and 3(C), to bond a ferrite core (201) and a lead wire (202), liquid bonding agent (203) such as liquid poste or liquid starch is used at the top of the hole (204) or all the hole (204) of the core (201). However, the filter illustrated in FIG. 3(A) has a characteristic of a high impedance (.vertline.Z.vertline.) along with a weak mechanical strength, on the contrary, the filter illustrated in FIG. 3(B) has a characteristic of a low impedance along with a strong mechamical strength. Therefore, it desires to develop a new filter having a high impedance as well as a strong mechanical strength, by a simple process.